1. Field of the Invention
This invention relates to a driving circuit for a DC motor controlled by Hall generators or more particularly to a driving circuit for a DC motor which does not need a control for keeping the revolution of the motor at a fixed speed.
2. Description of the Prior Art
Heretofore, the driving circuit for a DC motor controlled by Hall generator of the type having no constant speed control has been controlled not by controlling the Hall element input signals particularly, but by a fixed input. This causes a voltage to remain on the transistor receiving the Hall element input signals when a load is applied to the motor, and the current which runs through the field coil does not increase in proportion to the increase of the load torque. Therefore, this type of driving circuit has a drawback in that the motor speed-load torque characteristic is degraded because the current running through the field coil does not increase in proportion to load torque, and the relationship between load torque and motor speed is not proportional.
It was also attempted to control the motor speed by increasing the supply voltage. However, since this causes no change in the conductivity of the transistor receiving the Hall element input signals and since, as a result, the increased amount of supply voltage is applied across the emitter and collector of the transistor, the electric current running through the field coil connected to the transistor does not vary linearly with respect to the change in the supply voltage. Accordingly the speed of the motor does not vary linearly with respect to the change in the supply voltage and a great inconvenience is caused because it was impossible to control the motor speed at will.
In order to remove such defects, a method in which the transistor is saturated in advance was also considered. According to this method, however, since the driving transistor connected to the field coil of each phase becomes perfectly conductive pulsewise, phases with which driving currents run through each adjacent coil are superimposed and, in addition, since an electric current which rises linearly runs through the driving coils a magnetic field nonuniform to the rotor is generated.
As a result, this method has the drawback of generating irregular rotary torque, generating pulsed signals from the driving coil which act on each element of the driving coil as noises, and causing erroneous operation to occur.